However, your minutes roll over at AT&T and you can call any other AT&T customer for free all the time.

At T-Mobile, you have to add M2M service for $6.99 a month in order to call other T customers for “free.” And there’s no rollover.

T-Mobile is a better deal, dollar-for-dollar. The question (and real value) if you’re using AT&T is getting all you can out of rollover minutes, and calling your other AT&T peeps for free. With more customers than any other carrier in the U.S., AT&T probably has more of your friends on their network than any other carrier.